1. Field of the Invention
This invention relates to in situ recovery of shale oil, and more particularly to techniques for maximizing recovery of shale oil from a system of in situ oil shale retorts formed in a subterranean formation containing oil shale, while supporting overburden loads sufficiently to avoid substantial subsidence during retorting operations in the system of in situ oil shale retorts.
2. Description of the Prior Art
The presence of large deposits of oil shale in the semi-arid high plateau region of the Western United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which upon heating decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits have been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; 4,043,598; and 4,192,554, which are incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded for forming a stationary fragmented permeable mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous material to produce heat, combustion gas and combusted oil shale. By continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, and a residual solid carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process. The products of retorting are referred to herein as liquid and gaseous products.
It is desirable to maximize the amount of oil shale subjected to retorting within a region of formation being developed. To this end it is desirable to minimize the amount of formation excavated from each retort site when forming void volumes in preparation for explosive expansion. The mined out formation is excluded from the in situ retorting process, which can reduce the overall recovery of shale oil from the retorts. Removed formation either must be retorted by above ground techniques, or the shale oil is lost when the mined out material is discarded. Moreover, the steps of mining the shale and transporting it to above ground are expensive and time consuming.
It is also desirable to avoid significant uncontrolled subsidence in a tract of in situ oil shale retorts. There is a trade-off between extracting as much oil shale as possible to maximize resource recovery, and leaving sufficient unrecovered oil shale in the supporing pillars of unfragmented formation for supporting the weight of the overburden to avoid subsidence. Subsidence can result in fracturing of overburden with consequent leakage of water from overlying aquifers into retort or mining areas, leakage of gas from completed retorts, leakage of air into retorts during retorting operations, and safety hazards in underground workings containing operating personnel. Such subsidence can occur when the extraction ratio in the tract is large and the remaining unfragmented formation is not sufficient for supporting the weight of the overburden. For example, a fragmented mass having a substantial void fraction can have a substantially reduced compressive strength as compared with unfragmented formation. Because of the reduced structural support, subsidence of overburden can occur.
Techniques can be devised for developing a tract of in situ oil shale retorts so as to avoid substantial subsidence of overburden during the operating life of the retorts. However, a difficulty with a non-subsiding technique for developing a tract of in situ oil shale retorts is that total resource recovery can be limited, inasmuch as the oil shale that is left in unfragmented formation to support the overburden represents lost production.
Thus, it is desirable to provide a technique for developing a tract of in situ oil shale retorts without significantly uncontrolled subsidence of overburden, but also without leaving substantial proportions of oil shale in unfragmented formation at the end of all retorting operations.